The invention relates to an X-ray opaque glass, to a process for producing it and to its use.
Plastic dental compositions are increasingly being used for dental restoration in the dental sector. These plastic dental compositions usually comprise a matrix of organic resins and various inorganic fillers. The organic fillers predominantly comprise powders of glasses, (glass-)ceramics, quartz or other crystalline substances (e.g. YbF3), sol-gel materials or Aerosils.
The use of plastic dental compositions is intended to avoid possible harmful side-effects of amalgam and to achieve an improved aesthetic impression. Depending on the plastic dental compositions selected, they can be used for different dental restoration measures, for example for tooth fillings and also for securing parts, such as crowns, bridges and inlays.
The filling material per se is intended to minimize the shrinkage caused by the polymerization of the resin matrix during curing. For example, if there is a strong adhesion between tooth wall and filling, excessive polymerization shrinkage can lead to the tooth wall breaking. If the adhesion is inadequate, excessive polymerization shrinkage may result in the formation of peripheral gaps between tooth wall and filling, which can promote secondary caries. Furthermore, certain physical and chemical demands are imposed on the fillers:
It must be possible to process the filling material to form powders that are as fine as possible. The finer the powder, the more homogenous the appearance of the filling. At the same time, the polishing properties of the filling are improved, which in addition to reducing the surface area available for attack also leads to improved resistance to abrasion and therefore to a longer-lasting filling. To enable the powders to be processed successfully, it is also desirable for the powders not to agglomerate. This undesirable effect occurs in particular with filling materials produced with the aid of sol-gel processes.
Furthermore, it is advantageous if the filler is coated with functionalized silane, since this facilitates formulation of the dental composition and improves the mechanical properties.
Furthermore, the refractive index and color of the plastic dental composition and therefore also of the filler should be as well matched as possible to the natural tooth material, so that it is as far as possible indistinguishable from the surrounding, healthy tooth material. The grain size of the pulverized filler being as small as possible also plays a role in this aesthetic criterion.
It is also important for the thermal expansion of the plastic dental composition in the range of use, i.e. usually between −30° C. and +70° C., to be matched to that of the natural tooth material, in order to ensure that the dental restoration measure is sufficiently able to withstand temperature changes. An excessively high stress caused by temperature changes can likewise give rise to the formation of gaps between the plastic dental compositions and the surrounding tooth material, which in turn can form preferred points of attack for secondary caries. In general, fillers with the lowest possible coefficient of thermal expansion are used, in order to compensate for the high thermal expansion of the resin matrix.
A good chemical resistance of the fillers with respect to acids, alkalis and water and good mechanical stability under load, such as for example during the movement produced by chewing, can also contribute to a long service life for the dental restoration measures.
Furthermore, for the treatment of patients, it is imperative that dental restoration measures can be seen in an X-ray image. Since the resin matrix itself is generally invisible in an X-ray image, the fillers have to provide the required X-ray absorption. A filler of this type which provides sufficient absorption of X-radiation is described as X-ray opaque. Constituents of the filler, for example certain components of a glass, or additional substances, known as X-ray opacifiers, are generally responsible for the X-ray opacity. A standard X-ray opacifier is YbF3, which can be added to the filler in crystalline, milled form.
Because the plastic dental composition in use is usually introduced into cavities from cartridges and is then modeled in the cavities, it is often supposed to be thixotropic in the uncured state. This means that its viscosity decreases when pressure is exerted, whereas it is dimensionally stable without the action of pressure.
Among plastic dental compositions, a distinction also needs to be drawn between dental cements and composites. In the case of dental cements, also known as glass ionomer cements, the chemical reaction of the fillers with the resin matrix leads to curing of the dental composition, and consequently the curing properties of the dental composition and therefore their workability is influenced by the reactivity of the fillers. This often involves a setting process which is preceded by a radical surface curing, for example under the action of UV light. Composites, also referred to as filling composites, contain, by contrast, fillers which are as chemically inert as possible, since their curing properties are determined by constituents of the resin matrix itself and a chemical reaction of the fillers is often disruptive for this.
Because glasses, on account of their different compositions, represent a class of materials with a wide range of properties, they are often used as fillers for dental compositions. Glasses of this type are generally known as dental glasses. Reactive dental glasses for use in dental cements are known, for example, from DE 100 63 939 A1.
Chemically inert dental glasses for use as filler in composites form the subject matter of DE 198 49 388 A1. The glasses proposed therein must contain significant proportions of Al2O3, ZnO, F and Na2O, which has an adverse effect on their chemical resistance. Furthermore, the F, ZnO and Na2O contents can lead to reactions with the resin matrix, which can in turn have effects on their polymerization properties. In the glasses which form the basis of DE 198 49 388 A1, ZrO2 must also be present as a further constituent in order to effect X-ray opacity. Fillers of this type are too reactive in particular for the most modern epoxy-based dental compositions, in which excessively rapid, uncontrolled curing may occur.
DE 101 00 680 A1 has disclosed dental glasses which contain at least two components. These binary glass systems comprise a high content of SiO2 and HfO2, TaO5, ZrO2 or La2O3, which effect the X-ray opacity. However, only the binary glasses comprising SiO2 and HfO2 or SiO2 and Ta2O5 were produced by a melting process, whereas the glasses comprising SiO2 and ZrO2 and/or La2O3 were obtained with the aid of a sol-gel process. The sol-gel process has been described, for example, by Nogami in Journal of Non-Crystalline Solids, 96 (1985) 415-423.
In economic terms, it has the drawback of being too expensive for the production of relatively large quantities of dental glasses. Furthermore, glasses produced by the sol-gel process generally contain a large amount of water, making it difficult to process them further to form powders. In particular, sol-gel glass powders are often prone to agglomeration. Furthermore, DE 101 00 680 A1 proposes ternary glass systems which, in addition to SiO2 as the main constituent, also contain ZrO2 as X-ray opaque constituent and La2O3, HfO2, Y2O3, TiO2 and Al2O3 as further constituents. The ternary system comprising SiO2, La2O3 and B2O3 is also described.